Electroluminescent (EL) devices including an organic substance have been promising candidates for wide-area, full-color, and inexpensive display devices based on solid-state emission, and development of a variety of such devices is under way. Generally, an EL device is composed of a pair of electrodes, and a light-emitting layer interposed between the electrodes. Light emission is a phenomenon occurring through the following mechanism. When an electric field is applied between electrodes, electrons are injected from the cathode and holes are injected from the anode, both to the light-emitting layer. In the light-emitting layer, the injected electrons are recombined with holes, thereby creating an excited state. During transition from the excited state to the ground state, energy is released as light.
As compared with inorganic light-emitting diodes, conventional organic EL devices are operated at higher operation voltage and exhibit lower emission luminance and emission efficiency. In addition, organic EL devices are not actually used in practice, because of considerable impairment in characteristics. Recently, organic EL devices have been improved step by step. However, further improvement in emission efficiency and service life is demanded.
One disclosed technique is based on employment of a single monoanthracene compound serving as an organic light-emitting material (Japanese Patent Application Laid-Open (kokai) No. 11-3782). However, this technique is not practically employed, since luminance at a current density of 165 mA/cm2 is as low as 1,650 cd/m2, and emission efficiency is as considerably low as 1 cd/A. Another disclosed technique is based on employment of a single bisanthracene compound serving as an organic light-emitting material (Japanese Patent Application Laid-Open (kokai) No. 8-12600). However, emission efficiency attained by the technique is as low as about 1 to 3 cd/A, which remains to be improved before the technique is put into practice. Meanwhile, a long-life organic EL device has been proposed (WO 94/06157). The EL device includes a distyryl compound serving as an organic light-emitting material in combination with an additive such as styrylamine. However, the proposed EL device has an insufficient half-life, which is to be further improved.
Still another disclosed technique is based on employment of an organic light-emitting medium layer containing a mono- or a bis-anthracene compound and a distyryl compound (Japanese Patent Application Laid-Open (kokai) No. 2001-284050). According to the technique, a peak in an emission spectrum is red-shifted because of a conjugation structure of the styryl compound, thereby impairing color purity.